This invention relates to ventilation devices such as non-invasive positive pressure ventilation (NIPPV) and continuous positive airway pressure (CPAP) devices which function to supply a patient with a supply of clean breathable gas (usually air, with or without supplemental oxygen) at a prescribed pressure or pressures at appropriate times during the patient""s breathing cycle. The specification discloses a method and apparatus for fault diagnosis in such devices.
An example of a suitable device in which the invention may be included is the AutoSet(copyright) T device (ResMed Ltd., Australia), which may be used for treating sleep disordered breathing, such as Obstructive Sleep Apnea (OSA), as described in U.S. Pat. No. 5,704,345 (Berthon-Jones).
A NIPPV or CPAP device typically includes a flow generator, an air filter, a mask, an air delivery conduit connecting the flow generator output to the mask, various sensors and a microprocessor-based controller. The flow generator may include a servo-controlled motor and an impeller. The sensors measure, among other things, motor speed, gas volumetric flow rate and pressure. The air delivery circuit is that portion of the device""s airflow path comprising the air inlet, the filter, the flow generator, the conduit and the mask. The device may optionally include a humidifier in the air delivery circuit. The controller may include data storage means.
One problem compromising the effective operation of such devices is that with time, the air inlet filter may become dirty, increasing the filter""s resistance to the flow of air. It is known to provide a warning light which is activated after a given number of hours of operation of the device, to indicate that the filter should be cleaned or replaced. Such an approach does not take into account the fact that the rate of accumulation of dirt on the filter will depend on the environment in which the device is used. It may also be found that the patient may continue to use the device despite the warning light and thereby receive inadequate therapy. Importantly, the air delivery circuit may become partly or completely blocked for other reasons, without a warning being given.
At a given rotational speed of the motor, and a given air delivery circuit pneumatic impedance, the flow generator will deliver gas at a particular pressure. By measuring the resultant pressure for a range of motor speeds, for different circuit impedances, and with and without the humidifier, characteristic curves of the device may be obtained. If then the values of motor speed, flow rate and pressure are measured by transducers during operation of the device, the current pneumatic impedance may be calculated from the appropriate characteristic curve. It is therefore possible to estimate the resistance of the air filter and thus more accurately indicate when the filter needs changing. Furthermore, partial or complete blockage of the air delivery circuit will also be capable of detection by such an arrangement.
An acceptable range of measured pressure values may be calculated for a given set of conditions, defined, for example, by upper and lower characteristic curves. The motor speed may be increased to maintain an acceptable output pressure as a response to increased impedance, while a pressure outside an acceptable range may be indicative of the need to replace the filter.
Such an arrangement relies on the transducers providing correct information regarding the monitored parameters, and in accordance with preferred forms of the invention the existence of transducer fault conditions is also responded to by the system.
The invention therefore broadly resides in a method or apparatus in which, in each case, acceptable and unacceptable regions for transducer values are chosen. During operation of the device, the transducer values are measured and compared with the predetermined regions. When the transducers are in unacceptable regions, corrective action is taken, for example, by issuing a warning of the fault or shutting down the device.
Thus, in one form, the present invention provides apparatus for supplying breathable gas to a patient, including a gas flow generator, a gas delivery circuit, a controller having data storage means, sensors monitoring values of operational parameters of the apparatus, and fault detection means including at least one relationship stored in said data storage means, said relationship relating a combination of values of at least two of said parameters as indicative of a fault condition of said apparatus, said fault detection means further including means testing said at least two said monitored operational parameter values against said stored relationships and instigating a response upon detection of a fault condition.
Preferably, said monitored parameters include at least motor speed of the flow generator, gas flow rate and gas delivery circuit pneumatic pressure.
In the practice of this aspect of the invention, a device embodying the invention will before clinical use be subjected to a calibration routine in which the motor speed and air delivery circuit pneumatic pressure are varied, while measuring pressure and flow rate. From these measurements characteristic curves are determined and stored in the data storage means associated with the controller.
During clinical use, flow rate, pressure and speed are monitored. The appropriate characteristic curves and acceptable range of pressure are selected for the current set of operating conditions. The measured pressure is tested against the acceptable range, and if it lies outside that range, a fault is asserted. If a fault is asserted, an error message may be given on the output screen, an alarm given, or the machine may be shut down.
In other embodiments of the invention, all of the transducers (such as snore, flow and speed transducers) are tested against predetermined characteristic curves for fault conditions.
Further embodiments are described below in relation to CPAP apparatus, but will be understood as being applicable to any of the above described forms of ventilatory treatment or assistance.